Climate Less Sensitive to Greenhouse Gases Than Predicted, Study Says

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But some estimates over the past five years have suggested that the temperature change may be much higher.

Several studies have found that the temperature change may be higher than 16.2ºF (9ºC). One estimate put it at 19.8ºF (11ºC).

To obtain their estimate, Hegerl and her colleagues used reconstructions of the climate in the Northern Hemipshere over the past 700 years, which were made using data about ancient volcanic eruptions, changes in solar radiation, and greenhouse gas levels.

They then used a simple computer model to determine what type of climate conditions led to those temperatures, tweaking different variables such as volcanic ash and solar radiation.

On the basis of this model, they found a climate sensitivity of 2.2º to 15.5ºF (1.2º to 8.6ºC).

In addition, the team used a different model using climate data from only the 20th century and came up with a similar result.

"We have two lines of evidence, basically," Hegerl said.

When the team combined the two estimates, the researchers found that the climate sensitivity range narrowed even further to 2.7º to 11.2ºF (1.5º to 6.2ºC).

These findings are similar to the conclusions reached by Charney in 1979, rather than the more extreme ranges estimated in recent studies.

The results are reported in tomorrow's issue of the journal Nature.

Controversial Results

Gavin Schmidt is a climate modeler with the NASA Goddard Institute for Space Studies in New York. He said the study is the first to formally confirm scientists' assumptions about climate sensitivity.

"Basically no one really believes that those really high sensitivities [measured in the past five years] are possible," he said.

But Michael Schlesinger, a professor of meteorology at the University of Illinois at Urbana-Champaign and co-author of a 2001 paper on climate sensitivity, is wary of the new study's methodology.

In particular, Schlesinger questions the effects of comparing climate sensitivity based on the 20th century record with another estimate based on a 700-year record.

"If they chose a different [record]—if they chose our [study] or any others—and had done this, I do not see how [the range] gets narrowed," Schlesinger said.

"I'm not comfortable with the results."

On the other hand, James Annan, a climate modeler with the Frontier Research Center for Global Change in Kanagawa, Japan, said the sensitivity range should be narrowed even further, based on the results of his own work published in March.

"If they had looked at a greater range of evidence, then the limits would have been even tighter," he said.

Regardless of the numbers, most scientists seem to agree that the study confirms that the climate remains susceptible to global warming.

The top of the range found in Hegerl's study is higher than that found in 1979, Schlesinger points out.

"[That] means climate sensitivity is larger than we thought for 30 years," he said. "So the problem is worse than we thought. This doesn't give us any solace."

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